A uniquely stable trimeric model of SARS-CoV-2 spike transmembrane domain

The spike (S) protein of SARS-CoV-2 effectuates membrane fusion and virus entry into target cells. Its transmembrane domain (TMD) represents a homotrimer of -helices anchoring the spike in the viral envelope. Although S-protein models available to date include the TMD, its precise configuration was given brief consideration. Understanding viral fusion entails realistic TMD models, while no reliable approaches towards predicting the 3D structure of transmembrane (TM) trimers exist. Here, we propose a comprehensive computational framework to model the spike TMD (S-TMD) based solely on its primary structure. First, we performed amino acid sequence pattern matching and compared molecular hydrophobicity potential (MHP) distribution on the helix surface against TM homotrimers with known 3D structures and thus selected the TMD of the tumour necrosis factor receptor 1 (TNFR-1) for subsequent template-based modelling. We then iteratively built an all-atom homotrimer model of S-TMD based on "dynamic MHP portraits" and residue variability motifs. In this model each helix possessed two overlapping interfaces interacting with either of the remaining helices, which include conservative residues I1216, F1220, I1227, M1229, and M1233. Finally, the stability of this and several alternative models (including a recent NMR structure) and a set of mutant forms was tested in all-atom molecular dynamics (MD) simulations in a POPC bilayer mimicking the viral envelope membrane. Unlike other configurations, our model trimer remained extraordinarily tightly packed over a microsecond-range MD and retained its stability when palmitoylated in accordance with experimental data. Palmitoylation had no significant impact on the TMD conformation nor the way in which the lipid bilayer was perturbed in the presence of the trimer. Overall, the resulting model of S-TMD conforms to known basic principles of TM helix packing and will be further used to explore the complex machinery of membrane fusion from a broader perspective beyond the TMD.